code: def test_string_function_is_None(process_test_df):
      """Test that dataframe is returned if string_function is None."""
      result = process_test_df.process_text(column_name="text")
      assert_frame_equal(result, process_test_df)
  
imports: import numpy
  import pandas
  import pytest
  from pandas.testing import assert_frame_equal
label_desc: Use image node_id 9 for calling a global function with example usage: test_string_function_is_None(process_test_df) without return types